Safetying 101

The word safetying is a term universally used in the aircraft industry. Briefly, safetying is defined as: “Securing by various means any nut, bolt, turnbuckle etc., on the aircraft so that vibration will not cause it to loosen during operation.” These practices are not a means of obtaining or maintaining torque, rather a safety device to prevent the disengagement of screws, nuts, bolts, snap rings, oil caps, drain cocks, valves, and parts. Three basic methods are used in safetying; safety-wire, cotter pins, and self-locking nuts. Retainer washers and pal nuts are also sometimes used.

I want to discuss safetying when it comes to safety wire and the proper installation when it comes to groups of bolt heads, pins and bolt shanks. When bolts, screws, or other parts are closely grouped, it is more convenient to safety wire them in series. The number of bolts, nuts, screws, etc., that may be wired together depends on the application.

To prevent failure due to rubbing or vibration, safety wire must be tight after installation. The safety wire should always be installed and twisted so that the loop around the head stays down and does not tend to come up over the bolt head, causing a slack loop. (Reference FAA-8083-30 Airframe Handbook chapter 5)

Safety wire must never be overstressed. Safety wire will break under vibrations if twisted too tightly. Safety wire must be pulled taut when being twisted, and maintain a light tension when secured.

NOTE
When installing safety wire always consult the maintenance manual. Be sure to use the size and material wire specified in the appropriate maintenance manual.

When safetying two or three fasteners the direction of twist from the second to the third unit is counterclockwise in (reference AC 43.13-1b fig. 7-5 examples 1, 3, and 4 to keep the loop in position against the head of the bolt. The direction of twist from the second to the third unit in example 2 is clockwise to keep the wire in position around the second unit. The wire entering the hole in the third unit will be the lower wire, except example 2, and by making a counterclockwise twist after it leaves the hole, the loop will be secured in place around the head of that bolt.

The number of twits per inch in AC 43.13-1b fig. 7-3a shows 6-8 twists per inch. I think this is correct for most installations when using. However, 6 to 8 twists per inch by hand may not be appropriate of all safety wire installations. The recommended number twists per inch depends upon the diameter of the wire according to Aviation Mechanic Handbook, third edition by Dale Crane as follows:
Wire diameter Twists per inch
0.020 - 0.025 8 – 14
0.032 – 0.041 6 – 11
0.051 – 0.060 4 -9

For those that were trained in the military on safety wire the structural repair manuals state to keep the wire under tension twist in a clockwise direction until the wire is tight. When tightened the wire shall have approximately 8-10 turns per inch. (Reference T.O. 1-1A8 figure 16-3)

Keep in mind for U.S. type certificated aircraft the FAA uses 6-8 turns per inch, but does not indicate that the wire size makes any difference. In all cases in accordance with Title 49 U.S. Code part 43 §43.13 Performance rules (general).
(a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in §43.16.

Bottom line is check your current manufacture maintenance manual for type, diameter of safety wire and installation procedures. If there is no guidance in the manufacture manual in most cases you can default to AC 43.13-1b as acceptable practices.

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